Introduction

Calcium polysulphide (CaS x ) (CAS no. 1344-81-6), also known as “calcium sulphide (CaS)” and “lime sulphur”, naturally occurs as gypsum (CaSO4·2H2O) or plaster of Paris (CaSO4.1/2H2O) (Knauf et al. 1973; Klein et al. 1985) in the environment. Under required conditions CaS x (x = 2–7) can be produced by oxidation of hydrogen sulphide (H2S) and calcium oxide (CaO) (density 2.61 g cm−3) (Borgwardt 1984). Levchenko et al. (2015) identified a semi-industrialized method to synthesize CaS x solution with a Ca/S ratio of 10:23.2 at 100 °C in 200 l of distilled water resulting in 70–75 % CaS x yield. CaS x is a dark orange solution with a volatile and unstable nature, especially if exposed to air (Chrysochoou and Ting 2011; Levchenko et al. 2015). CaS x molecules can exist as chains of heptasulphide, octasulphide, and nanosulphide species with a pH value ranging from 6.0 to 11.0 (Gun et al. 2004; Kamyshny et al. 2007a, b). This long chain tends to breakdown into shorter chains as the pH decreases (Chen and Morris 1972). CaS x solution contains only 10 % of elemental sulphur at pH 12.0 in the form of polysulphides (S6 and S5) (Chrysochoou et al. 2010). Other species of sulphur exist in the CaS x solution such as hydrosulphide (HS−), thiosulphate (S2O3 −), dithionate (S2O6 2−) and H2S (Yahikozawa et al. 1978; Kelsall and Thompson 1993). Some sulphur species like S5 2−, S4 2−, and H2S becomes the dominant species at pH values less than 8.0 (Kelsall and Thompson 1993; Chrysochoou et al. 2010).

CaS x solution has a strong reducing capability towards heavy metals and radionuclides and acts by making precipitates with insoluble metal in the form of their sulphides (Jacobs 2001; Zhong et al. 2009). The minimum purity of CaS x is between 290 and 320 g kg−1 required by EFSA with boiling point of 104.8 °C and solubility in water above pH 8.5 (EFSA 2010; Table 1). CaS x is also regarded as one of the most promising reductants, with field applications in soils and for chromite ore processing residue (COPR) (Storch et al. 2002; IETEG 2005; Charboneau et al. 2006; Wazna et al. 2007; Bewley and Clarke 2010).

Table 1 Physical and chemical properties of CaS x in its pure form (EFSA 2010)
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CaS x is used as “Liquid-Lime Sulfur” (LLS) (EPA reg. no. 66,196–2) in agriculture where a typical solution contains a mixture of 29 % (w/v) CaS x and small amounts of calcium thiosulfate (Smilanick and Sorenson 2001). LLS is prepared by the combination of hydrated lime (CaO·H2O) and elemental sulphur with water (Tartar 1914; Auld 1915).

The aim of this literature review is to combine all the available scientific papers on CaS x applications to provide a cross section of major research related to CaS x . The goal of this review is to summarize the physico-chemical properties of CaS x , its different applications, study its kinetics, and provide a critique on its environmental fate (Tables 1, 2, 3, and 4). The range of applications include metal and organic pollutant removal from the environment including air, water, soil, sediments, and COPR, which makes it an attractive chemical for research. The understanding of proper mechanism and kinetics of CaS x can provide help to researchers and stakeholders in its application. This review combines available scientific research and suggests the harmful effects of CaS x residues in the environment, which is commonly ignored by stakeholders.

Table 2 Application of various CaS x species at variable pH ranges
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Table 3 Summarized doses of CaS x with Potentially Toxic Elements (PTEs) analyzed by different techniques and methods
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Table 4 Summarized acute toxicity values of wildlife and aquatic biota
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Methods

This article covers peer-reviewed research papers, case studies, conference papers, and reports from EU government as well as United States Environmental Protection Agency (USEPA) dating as far back as 1914 to 2015. This review intends to combine standard techniques and methods published in peer-reviewed journals, summarize the supporting data from available research and present the findings in tabular form (Tables 1, 2, 3, and 4). The standard toxicity testing results from USEPA and European Food Safety Authority (EFSA) and the peer-reviewed articles were used to discuss environmental fate and risk assessment of CaS x (Table 4).

Applications of CaS x solution

Application of CaS x in agriculture

Since the nineteenth century, CaS x solution has been used to control fungus and molds (McCallan 1967; USEPA 2005; Table 2). More recently, CaS x solution was used as an active ingredient for insecticides in agricultural industries (Aratani et al. 1978b) and in the form of LLS for spraying trees, fruits, and rose bushes (Ganczarczyk et al. 1985). CaS x is known to reduce up to 80 and 70 % of the mold caused by Penicillium digitatum and Geotrichum citri-aurantii, respectively (Smilanick and Sorenson 2001). In a similar study by Palou et al. (2002), CaS x was studied with a range of other chemicals in order to select the most effective fungicide agent against P. digitatum or Penicillium italicum. The effectiveness of the selected chemicals was tested either alone or in mixtures at different temperatures. The results suggested that CaS x was more effective as a mixture of potassium sorbate (0.1 M) and sodium propionate (0.1 M) at pH 8.3 and potassium sorbate (0.1 M) and sodium acetate (0.1 M) at pH 7.0 compared to 8.5 M CaS x solution at 6.0 pH.

LLS is characterized by yellow-orange liquid, pH 11.5, with a density of 1.26 g mL−1 (Smilanick and Sorenson 2001). For a common recommended fungicide application, LLS solution is diluted to 3 % (w/v) at pH 10.0. The present lime sulphur formula was standardized in England and LLS was rediscovered in 1800 and applied as a pesticide spray in 1850 shortly after a fungi disease grape powdery mildew introduced from North America, devastate vineyards in Europe (Large 1940). By 1900, LLS was commonly used in California for apple scab, powdery mildew, San Jose scale, aphids, mites, brown rot of peaches, and other diseases (Tweedy 1967; Eckert and Eaks 1989). LLS was found highly active against the gray mite, Calacarus citrifolia in South Africa (Dippenaar 1958). At present, LLS is certified as an acceptable pesticide by most “organic” grower organizations and the United States Department of Agriculture (USDA). Reports of its applications as a control of postharvest diseases includes Thurston County review summary report (2012) by USEPA (2005). In this report, LLS was used as a fungicide-insecticide/miticide (spider mites) against Powdery mildew, anthracnose, and scab (USEPA 2005).

The mode of action is described as metabolism disruption and acts as a protectant by utilizing the fungicidal properties of elemental sulphur during its degradation (IUPAC 2009). Some studies suggest that LLS works by producing two toxic components, H2S and elemental Sulfur. H2S is an inhibitor of cytochrome oxidase and elemental Sulfur oxidizes cytochrome b to cytochrome c with a byproduct release of H2S (Smith et al. 1977; Dorman et al. 2002).

Among various energy metabolism enzymes, cytochrome oxidase also plays an important role in regulating xenobiotics in insects (Pant 1958). Smilanick and Sorenson (2001) reported, that LLS deposits on P. digitatum spores, even after repeated rinsing. This causes long-term sub-lethal growth inhibition by persistent residual deposits, suggesting that LLS works in a continuous persistent way to inhibit the growth of P. digitatum instead of killing them immediately. Haller (1952) reviewed, LLS use on stone fruit and found that applications before harvest substantially reduced the postharvest incidence of brown rot of peaches, caused by Monilinia fructicola. Poulos (1949) reported, postharvest applications of LLS to the peaches reduced postharvest brown rot incidence about 60 % without injury to the fruit. The LLS solution needs continuous maintenance and renewal because it is constantly releasing small amounts of H2S gas that acidifies the solution (Smilanick and Sorenson 2001; Hojjatie et al. 2011).

Application of CaS x for detoxification of organic compounds

Another application of CaS x is to remove polychlorinated organic compounds from fly ash (Tabata et al. 2013; Table 2). Fly ash can be produced by coal combustion in electricity generation process, flue gas treatment, and municipal and industrial waste incineration (Nriagu 1988; Sun et al. 2010; Kalogirou et al. 2010; Tabata et al. 2013). In a study by Tabata et al. (2013), aqueous mixture of calcium hydroxide (CaOH) and sulphur was used to reduce dioxin compounds such as polychlorinated dibenzo-p-dioxins (PCSDDs), polychlorinated dibenzofurans (PCDFs), and polychlorinated biphenyls (PCBs) generated from industrial and municipal waste incinerators. A rapid hydrodechlorination of chlorobenzene and PCBs derivatives has been reported after CaS x application. The study concluded that about 87 % of the PCDD/Fs and PCBs were decomposed and detoxified at temperature 150 °C with 30- to 60-min agitation.

Application of CaS x for detoxification of metals

Detoxification of Cr(VI)

Chromium is one of the toxic and carcinogenic heavy metals frequently detected in polluted areas with Cr metallurgy, leather tanning, and metal plating industrial facilities (Syracuse Research Corporation 1993; Katz and Salem 1994; Chen et al. 1996; Chandra et al. 1997). Chromium occurs in various oxidation states ranging from Cr (−II) to Cr(+VI) out of which Cr(VI) and Cr(III) are the most stable species in the environment (Zayed and Terry 2003). Cr(VI) is mobile in soils with acidic and alkaline pH (Adriano 1986). To remove Cr(VI) from polluted area, it is reduced to Cr(III) in the presence of various sulphides, ferrous iron and soil organic matter (Rai et al. 1989). CaS x has been used as a rapid reductant of soluble Cr(VI) to immobile Cr(III) and this reaction has been referred to as “geochemical fixation” (Fruchter 2002; Freedman et al. 2005).

Chyrosochoou et al. (2010) treated Cr(VI)-contaminated glacial soil from a Cr plating facility using 1× and 2× stoichiometric ration of CaS x and analyzed the resultant Cr(VI) with synthetic precipitation leaching procedure (SPLP) and micro X-ray fluorescence spectrometry (Micro-XRF). The pH of treated soil increased from 6.0 to 11.0 upon CaS x addition and returned to 8.0–8.5 after 1 year. This suggests that CaS x created an apparent Cr(VI) immobilization environment for the first 60 days of treatment which subsequently decreased the solubility of Cr(VI) for up to 1 year of monitoring. Similarly, studies reported, CaS x as an effective agent to reduce Cr(VI) to Cr(III) at a former wood treatment facility (Jacobs 2001; IETEG 2005). Storch et al. (2002) reported Cr(VI) reduction at a former chrome plating facility in Arizona using CaS x . One of the case studies at Morsen pond Culvert, MA, USA, suggests the successful application of CaS x to treat Cr in a railroad embarkment with Cr laden pigment. CaS x also applied in Hanford cite to remove Cr from soil (Chaboneau et al. 2006).

CaS x solution has been used for the treatment of wastewater, groundwater and effluents attributable to different industries containing high concentrations of toxic elements (Aratani et al. 1978a, 1978b; Yahikowaza et al. 1978; Takaoka and Ganczarczyk 1985; Messer et al. 2004; Chen et al. 2009; Table 2). CaS x removed Cr contamination by reducing Cr(VI) to Cr(III) within 30 min of CaS x treatment (Yahikozawa et al. 1978). Cr(III) can be precipitated at alkaline pH to Cr(OH)3 (Graham et al. 2006). CaS x solution also studied in relation to the treatment of Cr by ion-exchange method in waste brine solutions (Pakzadeh and Batista 2011). Authors concluded that a molar ratio of CaS x to Cr(VI) between 0.60 and 1.40 at pH range 8.00–10.30 is needed to obtain a final Cr concentration less than 5 mg l−1.

Messer et al. (2004) recommended CaS x as a permanent remediation technique for Cr(VI) fixation in soil (alluvial fan sediments) and groundwater, based on a study at a former metal plating facility in Western Arizona. In this study, approximately 660 gal of 29 % CaS x was applied to the 20-ft2 test zone of infiltration trenches for 24 h followed by 2500 gal of water to disperse the chemical through test zone. Results for the first 30 days indicated a 90 % reduction in Cr(VI) concentration. In the same study, authors applied approximately 9000 gal of 29 % CaS x in an aquifer at about 165 ft below the surface of the soil followed by 79,000 gal of water. In this case, less than 1 mg l−1 Cr(VI) was observed after 35 h of CaS x treatment.

There were attempts to treat groundwater contaminated with Cr(VI) in anaerobic conditions (Zhong et al. 2009). One study considered the foamability of CaS x and determined the reducing potential of CaS x when delivered as foam. CaS x was mixed with surfactant sodium POE(3) laureth sulphate, or sodium laurylether sulphate, to treat Cr(VI) in groundwater under anaerobic conditions. Delivery of CaS x as a foam was compared with the direct flushing of the CaS x solution. It was concluded that direct liquid flushing of CaS x solution was more effective than the delivery of CaS x foam (>98 % of total water leachable Cr(VI) was mobilized compared to 28 % mobilization, respectively). Another study reported the reduction of Cr(VI) in solution under anaerobic condition catalyzed with sulphur nanoparticles resulted in accelerated Cr(VI) reduction (Lan et al. 2005).

CaS x was also widely used as a strong reductant Cr(VI) to Cr(III) in COPR which is generated as a waste by-product during industrial chromite ore processing. COPR contains high amounts of Cr(VI) (Freese et al. 2014). CaS x is used for the treatment of COPR by reducing Cr(VI) to Cr(III) at variable pH (Lan et al. 2005; Graham et al. 2006; Farmer et al. 2006; Wazne et al. 2007; Moon et al. 2008, 2009; Chrysochoou et al. 2009a, 2009b, 2010; Jagupilla et al. 2009; Chrysochoou and Ting 2011; Freese et al. 2014). CaS x solution was directly applied on the COPR to treat the Cr(VI). The effect of the treatment was evaluated by measuring the resultant Cr(III) either quantitatively or qualitatively. In each of these studies, different molar ratios of CaS x solution to Cr(VI) were used under different pH conditions (Table 2). Graham et al. (2006) were first to use CaS x for the COPR treatment in Glasgow, UK, in groundwater contamination from the chemical works and suggested the pH range 8.0 to 12.5 was more effective.

CaS x requires suitable conditions to reduce Cr(VI) at different rates. COPR treatment is challenging in terms of speciation and distribution of Cr(VI) specially when Cr(VI) occurs in the solid phase. According to Chyrosochoou et al. (2009b) solid phase made Cr(VI) inaccessible for CaS x to bind. Therefore in order to react Cr(VI) with the added sulphur ions (CaS5), it is essential to release Cr(VI) from solid matrix into solution. Similarly, Cr(VI) was bounded as PbCrO4 in the beginning that precipitated in the interspatial pores of soil and had a long residence time in environment (Chyrosochoou et al. 2010). Authors also concluded that in situ reduction is not an efficient treatment method for high Cr(VI) containing soils in surficial layers. The CaS x application for Cr application has been summarized in Table 2.

Detoxification of other potentially toxic elements (PTEs)

Another application of CaS x is to remove PTEs from different matrices such as fly ash, soil, wastewater, and groundwater (Table 2). Fly ash generated from municipal waste incineration facilities contains high levels of pollutants like dioxins and heavy metals including Pb and Cr(VI) (Bosshard et al. 1996; Nagib and Inoue2000; Hartmut et al. 2001; Kalogirou et al. 2010; Darakas et al. 2013). In a study by Sun et al. (2010), the application of CaS x solution in fly ash precipitated Pb as PbO/PbCO3. It was concluded that there was an overall decrease in soluble Pb concentration from the leachates of treated fly ash by post-treatment leaching tests.

CaS x solution has also been used for the treatment of wastewater, groundwater and effluents attributable to different industries containing high concentrations of toxic elements (Aratani et al. 1978a, 1978b; Yahikowaza et al. 1978; Takaoka and Ganczarczyk1985; Messer et al. 2004; Chen et al. 2009; Table 2). Yahikowaza et al. (1978) utilized CaS x (x = 4.7–5.4) as a single coagulant, to remove heavy metals (Hg2+, Cd2+, Pb2+, Cu2+, Zn2+, Cr3+, and Cr6+) from wastewater. CaS x also used as a potential treatment of industrial effluents containing concentrated cyanide (CN) liquors in combination of other polysulphides (Takaoka and Ganczarczyk1985). The study suggested that the treatment was effective in a broad range of CN concentrations and metal complexes.

Calcium sulphide (CaS) was studied in relation to the sulphidation of Zn, Ni, and Cu from wastewater originating from a plating facility (Soya et al. 2010). In this study, selective precipitation of metal sulphides achieved at different pH values and CaS x was described as the most effective precipitation agent for separation and recovery of Cu, Zn, and Ni. Increasing the molar ratio of the filtrates resulted in increasing pH value. In another study, CaS x solution was used for the treatment of automotive wastewaters containing Cu, Ni, Pb, and Zn (Kim et al. 2002). In this case, precipitation of metals as sulphides were compared to metal hydroxide precipitation in the presence of chelating agents and it was concluded that the presence of small amounts of chelating agents can decrease the solubility of metal sulphides. Chelating agents, such as EDTA, can inhibit the precipitation of Ni, Zn and Pb above the proposed standards. In a study by Mihara et al. (2008), CaS was produced as a recycled product of waste gypsum. CaS was then used for treatment of Ni containing simulated wastewater. The produced CaS was utilized as a sulphuration agent to treat simulated wastewater containing Ni. CaS to Ni ratio of 1.30 was successfully decreased the concentration of Ni from 100 mg l−1 to less than 1.0 mg l−1 (Table 3).

CaS x is also used for demercurisation processes as patented by Russian institution. In this process, a mercury containing waste, phosphor (the content is Tuti was determined according to GOST C 51768–2001) was mixed in polyethylene containers with the sample oxidizer (chlorine, lime, white, chloramine). To obtain a thick concrete solution consistency, 25–50 wt% water was added to the mixture and left for 7–8 h. CaS x (2.5 to 12 %) was added to this mixture at room temperature and laid out on pallets for 2–2.5 h until it dried out. Analytical analyses demonstrated a significant reduction in Hg concentration (0.0022 mg/m3) (Patent RU(11) 2 400 545(13) C1).

Application for CaS x as nanoparticle

Advancing technology and increasing consumer demand resulted in the development of efficient and more effective chemicals in the form of nanoparticles. One such chemical is sulphur nanoparticles which can be prepared from aqueous solution of CaS x with hydrochloric, oxalic, nitric, and sulphuric acid (Massalomov et al. 2014). Sulfur nanoparticles produced by polysulphides resulted in high-efficiency fungicide and plant growth regulators (Massalimov et al. 2013, 2014). Nanoparticle preparation from aqueous solution of polysulphides of alkali and alkaline-earth metals is described simple and ecologically safe (Massalimov et al. 2012). It is also interesting to note that nanoparticles prepared from CaS x with a mixture of hydrazine hydrate and monoethanolamine resulted in an average particle size of 20–25 nm, which further coarsen to 100–1000 nm after 10–15 min (Massalimov et al. 2014).

Technical issues associated with the molar ratio of CaS x required for treatment of PTEs

The available literature where CaS x solution was applied for environmental remediation primarily comprises case studies involving the remediation of COPR and examined the kinetics of CaS x reaction in greater depth (Graham et al. 2006; Wazne et al. 2007; Moon et al. 2008; Tinjum et al. 2008). There was little insight into the mechanism of CaS x reactions with other PTEs. However, there have been empirical findings regarding the stoichiometric ratio of CaS x to PTEs for an effective removal of PTEs, especially for Cr(VI). The ratio of CaS x to Cr(VI) depends on many factors such as; type of matrices (soil, water, COPR, or sediments), concentration of PTEs, aerobic or anaerobic and the pH conditions. Most of the available literature working ratio of CaS x solution with PTEs and their methodological approaches has been summarized in Table 3.

Kinetics of CaS x

Kinetics of CaS x studied with methylation of polysulphide suggests the higher concentration of hepta- and octasulphides whereas, lower concentration of Nona-, deca- polysulphides (Kamyshny et al. 2004). Yahikozawa et al. (1978) studied the kinetics of CaS x in wastewater with the presence of heavy metals (Hg2+, Cd2+, Cu2+, Zn2+, and Cr6+), O2 and CO2. The simple reaction in the presence of O2 and CO2 is provided as:

$$ {CaS}_x+3/2\ {\mathrm{O}}_2\to {CaS}_2{\mathrm{O}}_3+\left(x-2\right)\mathrm{S} $$
(1)
$$ {CaS}_x{+CO}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CaCO}}_3+{\mathrm{H}}_2\mathrm{S}+\left(x-1\right)\mathrm{S} $$
(2)

Equation (2) suggests the presence of H2S in CO2 contained solution. CaS x decomposes rapidly in the presence of CO2 compared to O2 and produce H2S, CaCO3, CaSO4, and solid sulphur particles. In wastewater containing heavy metals, the precipitating reactions can be explained as:

$$ {\mathrm{M}}^{2+}{+CaS}_2{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}\to MS+{\mathrm{CaSO}}_4+2{\mathrm{H}}^{+} $$
(3)
$$ {\mathrm{M}}^{2+}+{\mathrm{H}}_2\mathrm{S}\to MS+2{\mathrm{H}}^{+} $$
(4)

Where;

M represents the heavy metal cations

MS represents the precipitated metal sulphides.

Hg removal from soil and other open surfaces described by a Russian patent (Patent RU(11) 2 400 545(13) C1) follows the following reaction without CO2 and O2:

$$ {CaS}_x{+nHg}^{2+}=xHgS\downarrow {+Ca}^{2+} $$
(5)
$$ CaSx+nHg0+\left[\mathrm{O}x\right]=xHgS\downarrow {+Ca}^{2+} $$
(6)

According to the batch test experiment, Eq. (3) was found to be faster compared to equation (4). The precipitation rate was determined by the CaS x decomposition in former reactions (steps 1 and 2). The reactions from Eqs. 1 to 4 evolve with the formation of precipitate and known as “heterogeneous gas-liquid reaction”. Similarly Graham et al. (2006) suggested the following redox equilibrium reaction for COPR treatment using CaS x

$$ {2CrO_4}^{2-}{+3CaS}_5+10{\mathrm{H}}^{+}\to 2\ Cr{(OH)}_3+15\mathrm{S}\operatorname{o}{+3Ca}^{2+}+2{\mathrm{H}}_2\mathrm{O} $$
(7)

Tabata et al. (2013) reported the mechanism based on hydrodechlorination in which they used CaS x to reduce fly ash waste. The authors postulated that Ca(OH) and sulphur reacted to produce CaS x . The CaS x solution decomposed to provide a mixture of Ca(SH)2, Ca(OH)2 and Ca(SH)(OH) in the presence of water, which further produces CaS and CaS x . The resultant CaS and CaS x acted as strong reducing agents in the reaction mixture and reduced chlorinated compound to CaSO4 or CaSO3.

Ganczarczyk et al. (1985) studied the thermodynamic reaction of CaS x conversion to CN, resulted in thiocyanate production. In this study, a rapid moderately exothermic reaction of CN with CaS x was detected, where CN concentration was non-detectable within 2 weeks. CaS x was used for phosphate removal by precipitating phosphate ions into calcium phosphate present in wastewater in the presence of calcium ions (Aratani et al. 1978b). Similarly, LLS solves the water contamination problem by forming irreversible and insoluble sulphide salts with Zn and Cu (Salvato1992).

The environmental fate and toxicity aspects of CaS x

CaS x has a very high pH (about 11.5 which is corrosive) and can cause irreversible eye damage (EPA Toxicity Category I) and is considered mildly irritating to skin (EPA Toxicity Category III) (USEPA2005). Chronic dietary toxicity hazard, carcinogenicity, and mutagenic content have not been identified so far (USEPA 2005). CaS x exposure to midge eggs caused embryogenesis although evaluation of the testing methods indicates that the response may not cause by the CaS x ; therefore, endocrine disruption needs further research (Thurston county report 2012). The primary safety hazards it poses include burns from skin or eye contact with concentrated solution, or from exposure to H2S gas that can evolve from LLS. Occupational health and safety administration workplace limits for H2S are 10 μl l−1 for 8 h and 15 μl l−1 for brief exposures of 15 min (Anon 1997).

As discussed earlier, CaS x is used widely as a pesticide and insecticide in agriculture industries. About 500,000 tons of insecticides are now applied each year in the USA alone and insects such as aphids continue to affect agricultural productivity by developing resistance against these chemicals (van Emden and Harrington 2007). Pesticide resistance is the adaptation of pest populations resulting in decreased susceptibility to that chemical. In other words, resistance is the key mode of survival which evolves over time and gets genetically passed on to the offspring (PBS 2001). Resistance can be developed in insects with time and continuous usage of particular chemical. After 100 years of LLS usage, Melander (1914) reported orchard pests, the San Jose scale (Quadraspidiotus perniciousus) in the state of Washington, developed resistance to LLS on various in situ experiments. In the first experiment, all scales were killed with lime treatment within 1 week in typical orchards. However, the survival rate increased to 90 % after 2 weeks in an orchard with resistant scales. In a second experiment, the author used 2° (1 lb.: 7 gal), 3° (1 lb.: 5 gal), 5° (4 × 3°), 1:1/2:5, 1:2:5 factory made sulphur-lime in Clarkston, Walla Walla, Kennewick, Prosser, Sunnyside, North Yakima, and Wenatchee, USA, to carry out experiments with 350,000-scale insects. In a biweekly experiment, he found 50 % of the scales were still alive under the crust of dried spray. LLS also reported damage to fruit quality as well as reduced the number of harvested fruits with increasing application due to its sulphide content (Palmer et al. 2003). Authors reported a delay of maturity and fading of colors in apples treated with 12 % lime sulphur as well as decreased photosynthesis activity in plants.

Another application of CaS x is metal binding which makes it attractive for cleaning contaminants from wastewater and groundwater facilities. However, water based CaS x applications are not suitable for deep vadose zones and can lead to groundwater contamination by mobilizing the contaminant through preferential pathways within the sediments (Dresel et al. 2011). The Deep Vadose region is a zone of low permeability, having preferential flow area that is determined by gravity (Zhong et al. 2009).

Maletić et al. (2015) used many ameliorants to treat metal-contaminated soils, where CaS x solution was one of the ameliorants tested for its toxicity. The toxicity test was performed on the extracted pore water samples after treatment with different doses of CaS x solutions. For this purpose, a genetically modified lux-based biosensor: Escherichia coli HB101 (pUCD607) was used in a dose-response manner. The luminescence was measured after 15-min exposure. It was concluded that CaS x solution outperformed the other ameliorants. Furthermore, CaS x demonstrated the least toxic response as measured by the biosensor compared to other ameliorants studied (namely bone meal, activated carbon, and bentonite). Similarly, Bailey et al. (2012) exposed microbes, Shewanella oneidensis MR-1 to CaS x and other surfactants (sodium laureth sulphate (SLES), sodium dodecyl sulphate (SDS), cocamidopropyl betaine (CAPB), and NINOL40-CO). CaS x was found to be toxic at all the tested concentration (1.45–7.25 mM) and stimulatory at lower concentrations (20–500 μM). Most of the standard toxicity tests performed by IUPAC and USEPA used the LLS form. LLS demonstrated moderate acute toxicity in mammals, fish, and aquatic invertebrates and low acute toxicity in insects (Anon 1997; IUPAC 2009). Combined data with organisms is summarized in Table 4.

Conclusion

CaS x is an inorganic chemical that gained popularity in agricultural industries due to its effective insecticide and pesticide properties. It is most commonly used in the form of LLS and applied towards post-harvest treatments. With increasing application, recent technologies has developed sulphur nanoparticles from CaS x , which demonstrated a higher efficiency and improved regulation in plant growth. Although increasing use of any chemical can cause resistance issues in insects, limited research is available, making it difficult to identify the insect species which have developed resistance and the amount needed for successful application. There is a need of experimental series describing ecological issues regarding LLS for the effective measurements.

CaS x chemical properties to clean air, water, and soil pollution make it a prominent chemical in treatment of wastewater, groundwater and superfund sites. Its easy availability, economic cost, and successful application also adds value towards its industrial applications (Kameshwari et al. 2015). The hydrophobic nature of CaS x makes it a very applicable coagulant against heavy metal pollution from the environment. The kinetics demonstrated that the mode of action highly depends on pH, its physical state, and accessibility. The ability of CaS x to react with other metals and make them less soluble, nontoxic sulphides and hydroxides made it an approved reductant to clean contaminated land, wastewater, and groundwater (Jacobs 2001; Lewis 2011).

While a range of PTEs such as Zn, Pb, Fe, and Ni can be treated with CaS x ; however, its area of major application is Cr(VI) removal from groundwater, wastewater, soil, and COPR sources. CaS x is very effective and economically feasible to remove Cr(VI), which makes it popular in Cr plating and metallurgy industries. It works by precipitating heavy metals at certain basic pH. According to the literature reviewed so far, the chemical nature of CaS x makes it a very applicable chemical against of heavy metal pollution in the environment. Further successful applications in different industries can be identified by kinetics described within this paper.

Even after a range of use and application of CaS x , their molar ratio, dosage, and required chemical properties to work effectively is not very well defined. The successful application at defined pH is also missing in various prominent researches (Table 2). Limited stoichiometry data and kinetics with each metal makes it hard to implement its successful application with various heavy metals.

The limited research on the fate of CaS x in environment makes it a threat to living biota. Some government reports and peer-reviewed articles identified CaS x toxicity in various biota at certain quantity (Table 4), which requires attention and further studies. The lack of toxicity information cannot be compensated by field application and limited database reports by individual agency. The physiochemical properties of every single organism are different and highly depend upon their surrounding environment. Real-time lab experiments are necessary to identify its effects on living organisms in soil and water. Dietary as well as exposure experiments with organisms are required to define LC50 and LD50 values in living organisms.

CaS x application ranges from agriculture, metal refineries, leather tanning to construction industries. The wide range applications further expose it to the environment. So far, there is no documented human health effects associated with CaS x , which makes it an ecofriendly chemical against heavy metal pollution and other industrial application. However, there is a need to conduct some in depth studies related to the toxicity and resistance to avoid any future health and environmental hazards.